1. Field of the Invention
The present invention relates to an active matrix liquid crystal display and, more particularly, to a miniaturized active matrix liquid crystal display having high reliability.
2. Description of the Related Art
An active matrix liquid crystal display uses a liquid crystal as a display medium. A pixel is disposed at each intersection in a matrix construction. Every pixel is equipped with a switching device. Information about the pixels is represented by turning on and off the switching devices. Three-terminal devices which are thin-film transistors having gate, source, and drain are most often used as the switching devices. The thin-film transistors are hereinafter often abbreviated as TFTs.
In the matrix construction, scanning lines (gate lines) extending parallel to a row are connected with the gate electrodes of the TFTs in this row. Signal lines (source lines) running parallel to a column are connected with the source (or drain) electrodes of the TFTs in this column. A circuit for driving the scanning lines and a circuit for driving the signal lines are also provided.
FIG. 2 shows one conventional active matrix liquid crystal display. A signal line driver circuit 221 for driving signal lines is mounted above the pixel matrix 220 of this active matrix liquid crystal display. A scanning line driver circuit 222 for driving scanning lines is disposed to the left. Indicated by 223 is a sealant material region.
FIG. 3 is a cross section of FIG. 2. Pixel TFTs 311 are covered by a liquid crystal material 304. A liquid crystal material 304 is held between a TFT substrate 301 and a counter substrate 302. On the other hand, the signal line driver circuit 221 and scanning line driver circuit 222 are protected only by a thin film 309 of oxide or nitride. Therefore, TFTs forming these driver circuits are placed in a harsher environment than the pixel TFTs 311 located inside the liquid crystal material.
In an attempt to solve the foregoing problems and to obtain long-term reliability, a display device structure having improved reliability has been devised. In this structure, both signal line driver circuit and scanning line driver circuit are placed within a liquid crystal material, as well as pixel TFTs.
FIG. 4 shows a known device of this improved structure. In this known structure, a region of a sealant material or sealing material 423 is located outside both a signal line driver circuit 421 and a scanning line driver circuit 422. Therefore, the driver circuit TFTs are covered by the liquid crystal material, as well as the pixel TFTs. Furthermore, to miniaturize the liquid crystal display, three end surfaces (in FIG. 4, the top end surface, bottom end surface, and right end surface) of the counter substrate are made to conform to three end surfaces of the TFT substrate. Indicated by 420 is a pixel matrix.
These two conventional structures suffer from the following problems.
As shown in FIG. 5, in the conventional active matrix liquid crystal display, a short ring 506 is formed around the pixel matrix to protect the TFT devices from static charges. Since the signal lines 530 and scanning lines 531 connected with pixel TFTs 511 are all shorted, static charges produced during manufacturing steps, especially during rubbing steps, for the liquid crystal display are prevented from being applied across the terminals of each pixel TFT 511.
In the first-mentioned conventional structure shown in FIGS. 2 and 3, it is common practice to cut the short ring together with the glass substrate with a laser beam or the like in the final manufacturing step for the liquid crystal display.
However, in the second-mentioned conventional structure, in an attempt to minimize the size of the liquid crystal display, the counter substrate and the TFT substrate are preferably cut along common planes (in FIG. 4, the top end surface, bottom end surface, and right end surface of each substrate) from which no terminals are brought out. Accordingly, it is difficult to cut the short ring with a laser beam in the final step. In particular, the short ring is cut together with the substrate along a common plane. As shown in FIG. 6, after the cutting, the end surface of the TFT substrate 101 is exposed. If static charges are produced on the exposed end surfaces after the cutting, the internal pixel TFTs will be destroyed, thus making the display device defective.